It depends on whether you prune them strictly or let them get bushy. Plants which are pruned to one or two main stems and staked can be spaced 2 feet apart. If you cage them and let them go, 3 feet is needed. That's my opinion anyway. You may get by with 1/2 foot less than those numbers. Also, plants around the edge of your raised bed have room to expand beyond the bed. Just remember that you need access to the plants, so allow for that too.

Jim

As Jim said - anywhere from 2-4 feet depending on how you plan to support them and if you plan to prune or not. The Growing Tomatoes forum here (linked at the top of the page) has a FAQ on pruning and spacing and a search of that forum will pull up several discussions on plant spacing for you too. I linked some recent ones below.

There are also some discussions on companion planting there.

Dave

Here is a link that might be useful: Tomato Plant Spacing discussions...

I would say as little as 1 sq ft for indeterminate types pruned to a single stem, but you will get less yield from each plant (but may make up for it with more plants).

A determinate plant shouldn't be pruned and will need at least a 2'x2' area with 3'x3' probably better.

I use raised beds and space at 2 feet in cages. The beds are 30" wide. The cages are staggered in the bed so I get a little more distance. The rows are 2 feet apart and the tomatoes are in the last row. By planting deep I figure the tomato plants have lots of expansion room.

I tried to grow a better boy last year with a cage and a 5' x 5' space and it actually grew less tomatoes than my closer plants. Not sure why.

I usually go about 2 1/2 to 3 feet. Now If I could do it over I would build a 2 foot wide by whatever length raised bed for the tomatos. Then I would get cages that would always get tied down to the raised bed.